Various attempts have been made in the past to provide adequate lubrication for the bearings of spur gear pumps by bleeding off a portion of the fluid flowing through the pump and passing this portion through the pump bearings. For example, the pump wear plates have been provided with a metering slot extending between the shaft openings of the wear plate, in the zone where the gear teeth intermesh. In such a case, lubricant is forced in parallel via the metering slot through the bearings and then collected and returned to the pump inlet chamber. These long metering slots have the disadvantage that they weaken the seal plate so that high strength, expensive materials must be used. Also, the metering slots or notches are subject to both clogging and erosion which can seriously impair the distribution of lubricant. Moreover, air dissolved in the lubricant tends to be pulled out during flow through the metering notches, which leads to frothing of the lubricant and poor flow through the bearings. Finally, such parallel lubrication of the bearings requires rather large volumes of oil which do not reach the outlet port of the pump, thereby reducing overall efficiency.
Attempts have also been made to direct lubricant flow in series first to one bearing and then to the other, to reduce the overall volume of oil required for bearing lubrication. For example, it is known to provide a short metering slot which extends from the zone where the gear teeth intermesh toward only one of the gear shafts, on the high pressure side of the pump. In this case, lubricant is forced in series through one bearing via the short metering slot, through a channel in the pump housing to the other bearing, and then returned to the pump inlet chamber. Unfortunately, this prior art design is subject to several of the drawbacks noted previously for the parallel flow pump. Location of the metering slot on the high pressure side of the pump requires the use of a carefully sized slot to keep the bearing flow rates within limits as discharge pressure varies under load. Thus, the bearing flow is controlled by the system pressure and is difficult to regulate. Moreover, the metering slot is still subject to clogging and erosion due to its rather small size. As the volume between the gear teeth first decreases and then increases in such prior art designs, flow reversals are known to occur in the metering slot which can lead to less desirable lubricant flow patterns and lubricant frothing. Finally, location of the short metering slot on the high pressure side of the pump places a large pressure differential on the wear plate which tends to cause increased wear.